1. Field the Invention
This invention relates to electrical devices, particularly circuit protection devices, comprising PTC conductive polymer compositions
2. Introduction to the Invention
Conductive polymer compositions exhibiting PTC (positive temperature coefficient) behavior, and electrical devices comprising them, are well-known. Reference may be made, for example, to U.S. Pat. Nos. 3,243,753; 3,351,882; 3,861,029; 4,177,376; 4,237,441; 4,238,812; 4,255,698; 4,286,376; 4,315,237; 4,317,027; 4,329,726; 4,330,703; 4,352,083; 4,413,301; 4,426,633; 4,450,496; 4,475,138; 4,481,498; 4,534,889; 4,543,474; 4,562,313; 4,647,894; 4,647,896; 4,685,025; 4,654,511; 4,689,475; 4,724,417; 4,761,541; and 4,774,024; French Patent Application No. 7623707 (Moyer); European Patent Application No. 158,410; and commonly assigned, copending applications Ser. Nos. 141,989 (MP0715, Evans), now U.S. Pat. No. 5,049,850, published as European Application No. 38,713; 656,046 (MP0762, Jacobs et al), now abandoned in favor of four continuing applications, application Ser. Nos. 146,460 (now U.S. Pat. No. 4,845,838), 146,652 (now U.S. Pat. No. 4,951,384), 146,653 (now U.S. Pat. No. 4,951,382), and 146,654 (now U.S. Pat. No. 4,955,267), all filed Jan. 21, 1988, and published as European Application No. 63,440; 818,846, now abandoned, and 75,929 (MP1100, Barma et al) published as European Application No. 231,068; 83,093 (MP1090, Kleiner et al); now U.S. Pat. No. 4,861,966; 102,987 (MP1220, Fang et al) now U.S. Pat. No. 4,907,340; 103,077 (MP1222, Fang et al), now abandoned in favor of a continuation application Ser. No. 293,542, filed Jan. 3, 1989, now U.S. Pat. No. 4,924,074; 115,089 (MP0906, Fang et al) now abandoned; 124,696 (MP0906, Fang et al) now abandoned in favor of three continuation applications Ser. Nos. 455,715, 456,015, and 456,030, filed Dec. 22, 1989; 150,005 (MP0906, Fahey et al); now U.S. Pat. No. 4,780,598 and 219,416 (MP1266, Horsma et al), now U.S. Pat. No. 4,967,176.
Particularly useful devices comprising PTC conductive polymers are circuit protection devices. Such devices have a relatively low resistance under the normal operating conditions of the circuit, but are "tripped", i.e. converted into a high resistance state, when a fault condition, e.g. excessive current or temperature, occurs. When the device is tripped by excessive current, the current passing through the PTC element causes it to self-heat to an elevated temperature at which it is in a high resistance state. When the circuit protection device is "tripped", a thermal gradient is created. Where the thermal gradient flows in the same direction as the current flow, measures can be taken to assure that the peak temperature of the thermal gradient, i.e. the "hotline" or "hotzone" does not form near an electrode. Such preventative measures are described in U.S. Pat. Nos. 4,317,027 and 4,352,083, the disclosures of which are incorporated herein by reference.
Jakab U.S. Pat. No. 4,467,310, the disclosure of which is incorporated herein by reference, describes a battery feed resistor comprising a thick film resistor and a PTC resistor in the form of a disc. The thick film resistor and the PTC resistor are electrically connected in series and are mounted opposite each other on either side of the ceramic substrate carrying the thick film resistor, or on either side of another an insulating layer, with the objective of achieving close thermal coupling between the resistors. The purpose of this arrangement is to prevent the thick film resistor from posing a fire risk by becoming too hot when a fault causes power line voltage to be applied to the resistor. If such a fault occurs, the initial temperature rise of the thick film resistor heats the PTC resistor, which thus increases rapidly in resistance and reduces the current to a safe level before the thick film resistor becomes too hot. Thus the PTC resistor provides protection (both for the thick film resistor and for other components of the circuit) by reducing the current to a trickle current. In all of Jakab's devices the PTC resistor is composed of a ceramic material and is mounted so as to provide the closest possible thermal coupling between it and the thick film resistor.
Circuit protection devices which have improved physical properties and improved electrical performance are produced when the conductive polymer composition comprising the device is crosslinked. Such crosslinking can be accomplished through the use of chemical crosslinking agents or gamma or electron irradiation, or a combination of these. It is frequently true that ionizing irradiation generated by an electron beam results in the most rapid and cost-effective means of crosslinking.